Lithography is a technique for applying patterns to the surface of a workpiece, such as a circuit pattern to a semiconductor chip or wafer. Traditional optical photolithography involves applying electromagnetic radiation to a mask having openings formed therein (i.e., a transmission mask) such that the light or radiation that passes through the openings is applied to a region on the surface of the workpiece that is coated with a radiation-sensitive substance (e.g., a photoresist). Such traditional optical photolithography is reaching its resolution limit, however, due to the wavelength of electromagnetic radiation usable with transmissive masks.
An emerging candidate for finer resolution lithography uses Extreme Ultraviolet (EUV) light to image patterns on an area of a wafer. EUV light has a wavelength in a range of about 10 nm to 20 nm, in particular about 13.4 nm to 13.5 nm. EUV lithography (EUVL) employs reflective masks rather than transmissive masks since the EUV light at such a small wavelength is prone to be absorbed by materials used in a transmissive mask.
EUVL masks include a reflective film (e.g., a Bragg reflector) arranged on an ultra low expansion (ULE) substrate and a pattern of absorber material on the reflective film. The exposure light is incident on the mask at a shallow angle, e.g., about 5 or 6 degrees, relative to the perpendicular direction to the mask. Some of the incident light is reflected by the reflective film and some of the incident light is absorbed by the absorber material, thus producing a predefined pattern of light that is ultimately applied onto an area of a wafer, e.g., to expose a pattern in a photoresist on the wafer.
The pattern of absorber material and exposed portions of the reflective film are contained in an active area (also referred to as a primary pattern, pattern region, image field, etc.) of the EUVL mask. The EUVL mask also includes a border region (also referred to as a black border area) composed of an about 2-3 mm wide strip of absorber material that surrounds the active area. The same EUVL mask may be used many times in succession to provide the same predefined pattern of light on different areas (e.g., different dies) of a single wafer, and the border region is used to isolate the individual patterns as they are exposed on the wafer surface.
In order to provide desired pattern dimensional accuracy, the thickness of the layer of absorber material used in EUVL masks is typically constrained to be less than that which provides complete absorption of the incident EUV light. Thus, some of the incident EUV light is reflected even in areas of the mask covered by the absorber material. For example, the reflectivity of EUVL mask absorber material can range from about 1-3%.
This non-negligible absorber reflectivity in EUVL masks creates the potential for unwanted reflections from one exposure into the periphery of a neighboring exposure die. This is referred to as black border reflectivity and is a function of both absorber material reflectivity and placement of the reticle masking (REMA) blades that are used within the scanner to confine light to the active area. The reflective nature of an EUVL system requires the REMA blades be placed in front of the mask. To accommodate both a potential variation in the REMA blade placement and the fact that the REMA blade cannot be located in the mask image plane, a standoff distance between the REMA blade to the edge of the active area is provided. Thus, some incident EUV light impinges on the absorber material at the border region that surrounds the active area. This results in what is referred to as the black border reflection, where light reflected from this border region contributes unwanted intensity to a neighboring exposure die. Furthermore, because the REMA blade is not in the mask focal plane, a half-shadow, or penumbra, spatial signature to the black border reflection is observed.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.